A Root Box Method to Estimate Virulence of Helicobasidium mompa Using Carrots and Its Comparison with the Conventional Method Using Apple Stocks

Abstract

A root box method with carrots was developed to estimate virulence of the violet root rot fungus, Helicobasidium mompa, to facilitate short-term screening of many isolates during a year. The root box consisted of two transparent acrylic plates and a plastic bag of vermiculite in which two taproots of carrot were growing and inoculated with the fungus growing on fragments of mulberry twigs. The boxes were kept in a greenhouse at 25°C, and the surface of carrots was observed weekly up to 14 weeks. The virulence of each isolate was determined based on the number of weeks after inoculation required for the fungus to develop infection cushions on the surface of carrots. Results were compared with those from the conventional inoculation method using apple stocks. Two-year-old 456 apple stocks were planted with or without fungal inoculum in 30-cm diam. plastic pots containing commercial soil and placed outdoors in April 1999. Symptoms on plant tops were observed weekly, and the first stocks were killed 14 weeks after inoculation. At the end of trial 1 (6 months) and trial 2 (14 months), apple stocks were dug up to rate disease index (DI) based on hyphal growth and infection cushion formation on the stem base. There was variability in disease severity among replicates as well as isolate variability ; however, the results were similar in both trails. The level of virulence estimated by both methods was almost parallel for a total of 23 isolates from five plant species, except for two isolates from sweet potato that formed no obvious infection cushion on apple roots but on carrot were the most virulent.